One known traditional example of a technique for measuring the amount of minute deformation caused by, for example, pressing or heating an industrial product is speckle interferometry. FIG. 5 illustrates a speckle interferometry instrument described in Patent Literature 1. First, laser light ejected from a He—Ne laser light source 2 is reflected by a mirror 3, the reflected light is incident on a half mirror 6, and the light is separated to a direction in which a reference object 7 is disposed and a direction in which a specimen 3 is disposed. Light reflected off the surface of the specimen 8 (measurement light) passes through the half mirror 6 again and reaches a camera 11, whereas light reflected off the surface of the reference object 7 (reference light) is reflected by the half mirror 6 to the direction of the camera 11 and reaches the camera 11. The camera 11 can capture an image of an interference pattern having spots (speckles) caused by interference between the object light and reference light. When this speckle interference image is captured again after a certain period of time and the difference from the original speckle interference pattern image is determined, the extent of deformation occurring in the specimen 8 can be measured.
A technique of capturing an image of a grid pattern or moiré pattern projected by, for example, a projector onto a specimen and measuring the shape and deformation of the specimen from modulation of the pattern and a technique of measuring the shape and deformation of a specimen with high precision from an interference pattern using optical interference are commonly known.
These measurement techniques perform processing called sub-fringe analysis of analyzing the phase of an oscillatory waveform as which lightness and darkness of a captured image of a pattern is regarded. Because the phase of a waveform and the dimensions of the shape of a specimen are in proportion to each other, high-resolution shape measurement and deformation measurement can be achieved by detection of the phase from the captured image of the pattern with high precision.
For the above-described sub-fringe analysis method, the phase shift method is commonly known as a technique capable of analyzing a specimen with high resolution. The phase shift method is a method of first emitting original pattern light to a specimen, capturing its image, then capturing a plurality of images provided by emitting pattern light having the known phase variation, and calculating the phase of the image resulting from the original pattern light from a change in light and dark value in each point of the images.
At this time, in addition to the phase, amplitude and a background component are contained in the captured image as an unknown component. Therefore, to calculate phase information, it is necessary to simultaneously acquire at least two or more different phase shift images, in addition to an original image. Thus, a technique of disposing optical paths in which each of measurement light and reference light is separated to three portions, producing a phase difference by providing the optical path of the reference light with a refractive medium, and simultaneously acquiring a plurality of phase shift images is known (Patent Literature 2).
Citation List
Patent Literature
PTL 1 Japanese Patent Publication No. 7-46045
PTL 2 Japanese Patent Laid-Open No. 11-304417
However, for the invention described in Cited Literature 1, the amount of deformation is computed from a simple difference between intensity distributions of two speckle interference pattern images. Therefore, it is sufficient for the order of from micrometers to millimeters, but a situation exists in that resolution is insufficient for measurement of the amount of significantly minute deformation, such as one in the order of nanometers. Thus, clarification of a process of deformation of a specimen is limited.
For the technique described in Cited Literature 2, although resolution is sufficient, a plurality of optical paths for use in capturing a phase shift image for each lapse of time is required. Thus, the apparatus tends to be large and costly.
It is an object of the present invention to provide a deformation measurement method capable of measuring the amount of deformation of a specimen with high resolution at low cost without having to complicate an apparatus.